1. Field of the Invention
The present invention relates to a water-based ink with which recording can be made on not only coated paper, but also on non-coated paper generally used in offices and homes, i.e., so-called plain paper, at high printing density and quality. This invention further relates to an ink jet recording method using the ink, as well as equipment using the ink.
2. Description of the Related Art
An ink jet recording technique is advantageous in that less noise is generated during recording, and recording images of high resolution are obtained at a high speed by using heads with a high degree of integration. Such an ink jet recording technique employs, as a recording liquid, ink that is prepared by dissolving various water-soluble dyes in water or a mixture of water and an organic solvent. However, the use of water-soluble dyes often causes a problem in light resistance of recorded images because the water-soluble dyes are inherently poor in light resistance.
Owing to the fact that ink is water-soluble, there also occurs a problem in water resistance of recorded images. In other words, if recorded images are moistened with rain, sweat or drinking water, they are likely to blur or disappear.
Meanwhile, because of similar problems with respect to stationery usages involving such needs as ball pens employing dyes, various types of water pigment inks for stationery have been proposed with a view to solving the problems of resistance against light and water. From the standpoint of using water pigment inks, proposals for achieving dispersion stability, preventing ink from solidifying at ball points, and preventing abrasion of balls used in ball pens are discussed in, e.g., Japanese Patent Laid-Open No. 58-80368, No. 61-247774, No. 61-272278, No. 62-568, No. 62-101671, No. 62-101672, No. 1-249869, and No. 1-301760. Recently, ball pens, markers and the like using water-based pigment inks have been sold on the market. As ink jet recording ink using water pigment ink, Japanese Patent Laid-Open No. 56-147859, No. 56-147860, etc. propose ink using a particular water-soluble solvent and a high-molecular dispersant. Further, ink using both a pigment and a dye is proposed in Japanese Patent Laid-Open No. 4-57859, No. 4-57860, etc.
However, conventional water-based pigment ink has a disadvantage in that printing quality and density are remarkably deteriorated depending on the types of paper used, especially when recording is made on plain paper (such as copying paper and bond paper widely used in offices). In general, since the surface condition of plain paper is not uniform, unlike specific paper for ink jet recording, the quality of prints on plain paper varies depending on differences in absorption, spread and other properties of the ink. From the viewpoint particularly of printing density, which affects the quality of prints to a large extent, printing quality is less affected by ink using water-soluble dyes depending on the surface condition of paper as dyes are fixed to paper fibers and tend to easily stay on the paper surface. On the other hand, because water-based pigment ink establishes printing density by leaving pigments on the paper surface, the paper surface condition largely affects the printing density. More specifically, water-based pigment ink employs alkali-soluble resins as a dispersant in most cases so that printing density is increased by agglomeration of the resins on the paper surface. Although printing quality and density are remarkably deteriorated with some types of paper, it is thought that the use of alkali-soluble resins, especially (meth-) acrylic acid copolymers, is effective in not only satisfying printing quality and density, but also providing satisfactory dispersion of pigments and other properties. On the other hand, if those resins are used with ink for ink jet recording, many problems concerning reliability such as discharge stability, dispersion stability for a long term, and solidification of ink left at the fine nozzle end for a long time would be remained unsolved. This means that the use of those resins makes it difficult to achieve the desired printing density and quality and the desired reliability at the same time.
As means for improving printing density and quality, ink using alkali-soluble resins as a dispersant is disclosed in, e.g., Japanese Patent Laid-Open No. 56-147863, No. 56-147868, No. 56-157470, No. 60-223074 and No. 60-223075. However, the disclosed types of ink cannot provide satisfactory printing density and quality when used for ink jet recording.
Accordingly, there has been demand for an ink which can improve printing density and quality while meeting the reliability, required when used for ink jet recording, and particularly, stability in dispersion and discharge.